Neuronal circuits underlying acute morphine action on dopamine neurons.

Morphine is a highly potent analgesic with high addictive potential in specific contexts. Although dopamine neurons of the ventral tegmental area (VTA) are widely believed to play an essential role in the development of drug addiction, neuronal circuits underlying morphine action on dopamine neurons have not been fully elucidated. Here we combined in vivo electrophysiology, tract-tracing experiments, and targeted neuronal inactivation to dissect a neural circuit for acute morphine action on dopamine neurons in rats. We found that in vivo, morphine targets the GABAergic tail of the VTA, also called the rostromedial tegmental nucleus, to increase the firing of dopamine neurons through the activation of VTA μ opioid receptors expressed on tail of the VTA/rostromedial tegmental nucleus efferents. Our data also reveal that in the absence of VTA glutamatergic tone, there is no morphine-induced activation of dopamine neurons. These results define the anatomical organization and functional role of a neural circuit for acute morphine action on dopamine neurons.